Fix docs so WORKDIR mentions it works for COPY and ADD too

The docs around COPY/ADD already mentioned that it will do a relative
copy/add based on WORKDIR, so that part is already ok.  Just needed to
tweak the WORKDIR section since w/o mentioning COPY/ADD it can be misleading.

Noticed by @phemmer

Signed-off-by: Doug Davis <dug@us.ibm.com>

docs/man/Dockerfile.5.md

@@ -195,7 +195,7 @@ or
 
 **WORKDIR**
  -- **WORKDIR /path/to/workdir**
- The WORKDIR instruction sets the working directory for the **RUN**, **CMD**, and **ENTRYPOINT** Dockerfile commands that follow it.
+ The WORKDIR instruction sets the working directory for the **RUN**, **CMD**, **ENTRYPOINT**, **COPY** and **ADD** Dockerfile commands that follow it.
  It can be used multiple times in a single Dockerfile. Relative paths are defined relative to the path of the previous **WORKDIR** instruction. For example:
  **WORKDIR /a WORKDIR b WORKDIR c RUN pwd** 
  In the above example, the output of the **pwd** command is **a/b/c**.

docs/sources/reference/builder.md

@@ -801,8 +801,8 @@ and for any `RUN`, `CMD` and `ENTRYPOINT` instructions that follow it in the
 
     WORKDIR /path/to/workdir
 
-The `WORKDIR` instruction sets the working directory for any `RUN`, `CMD` and
-`ENTRYPOINT` instructions that follow it in the `Dockerfile`.
+The `WORKDIR` instruction sets the working directory for any `RUN`, `CMD`,
+`ENTRYPOINT`, `COPY` and `ADD` instructions that follow it in the `Dockerfile`.
 
 It can be used multiple times in the one `Dockerfile`. If a relative path
 is provided, it will be relative to the path of the previous `WORKDIR`